Recently, integration of a complementary metal oxide semiconductor (CMOS) device with a power device has been required for purposes such as mixed mounting of a power device and a control circuit thereof on a single chip. In this case, a lateral power MOSFET to which a high voltage is applied is formed on a CMOS chip having a fine structure. However, the lateral power MOSFET is required to have a high breakdown voltage, and therefore needs to be provided with a thicker gate oxide film than a gate oxide film of the CMOS. Meanwhile, the CMOS cannot have a thick gate oxide film for a high-speed operation. Nevertheless, when two types of gate oxide films are formed on a single chip, the number of manufacturing processes is considerably increased, and thus the manufacturing cost is increased.
Hence, there has been proposed a technique utilizing shallow trench isolation (STI) instead of forming a thick gate oxide film. For example, Document “J. Sonsky et. al., ‘Towards universal and voltage-scalable high gate- and drain-voltage MOSFETs in CMOS’ 2009 International Symposium on Power Semiconductor Devices and ICs, p. 315” discloses a technique by which: STI is formed in a p-type well; an opening of rectangular solid shape is formed in the STI; an n+-type source layer and an n+-type drain layer are formed in both end portions of the opening, respectively; and a gate electrode is formed on the STI. With this technique, a distance between the gate electrode and the well can be secured by arranging the gate electrode on an upper surface of the STI and arranging the well on side surfaces of the STI. This technique can form a lateral power MOSFET having a high breakdown voltage on a CMOS chip only by changing a mask layout without adding a process of forming a thick gate oxide film. However, studies by the inventors of the invention and others have revealed that there is a possibility that the device described in the aforementioned Document does not have a sufficient avalanche resistance and electrostatic discharge (ESD) resistance.